Date: Tue, 22 Sep 2020 13:37:18 +0000 From: Xen.org security team <security@....org> To: xen-announce@...ts.xen.org, xen-devel@...ts.xen.org, xen-users@...ts.xen.org, oss-security@...ts.openwall.com CC: Xen.org security team <security-team-members@....org> Subject: Xen Security Advisory 342 v3 (CVE-2020-25600) - out of bounds event channels available to 32-bit x86 domains -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Xen Security Advisory CVE-2020-25600 / XSA-342 version 3 out of bounds event channels available to 32-bit x86 domains UPDATES IN VERSION 3 ==================== Public release. ISSUE DESCRIPTION ================= The so called 2-level event channel model imposes different limits on the number of usable event channels for 32-bit x86 domains vs 64-bit or Arm (either bitness) ones. 32-bit x86 domains can use only 1023 channels, due to limited space in their shared (between guest and Xen) information structure, whereas all other domains can use up to 4095 in this model. The recording of the respective limit during domain initialization, however, has occurred at a time where domains are still deemed to be 64-bit ones, prior to actually honoring respective domain properties. At the point domains get recognized as 32-bit ones, the limit didn't get updated accordingly. Due to this misbehavior in Xen, 32-bit domains (including Domain 0) servicing other domains may observe event channel allocations to succeed when they should really fail. Subsequent use of such event channels would then possibly lead to corruption of other parts of the shared info structure. IMPACT ====== An unprivileged guest may cause another domain, in particular Domain 0, to misbehave. This may lead to a Denial of Service (DoS) for the entire system. VULNERABLE SYSTEMS ================== All Xen versions from 4.4 onwards are vulnerable. Xen versions 4.3 and earlier are not vulnerable. Only x86 32-bit domains servicing other domains are vulnerable. Arm systems as well as x86 64-bit domains are not vulnerable. MITIGATION ========== There is no known workaround for x86 32-bit Domain 0. The problem can be avoided by reducing the number of event channels available to 32-bit x86 guests to no more than 1023. For example, setting "max_event_channels=1023" in the xl domain configuration, or deleting any existing setting (since 1023 is the default for xl/libxl). CREDITS ======= This issue was discovered by Julien Grall of Amazon. RESOLUTION ========== Applying the appropriate attached patch resolves this issue. Note that patches for released versions are generally prepared to apply to the stable branches, and may not apply cleanly to the most recent release tarball. Downstreams are encouraged to update to the tip of the stable branch before applying these patches. xsa342.patch Xen 4.14 - xen-unstable xsa342-4.13.patch Xen 4.10 - 4.13 $ sha256sum xsa342* 8e85719f2783d5d0fc3da7a6aefb6c83717c7aa195d027b6aa52ff3a31c489aa xsa342.meta 060caee3fb5971fca0f2fbdef622c52d9bc6e0ed9efad33de5b6b504651c2112 xsa342.patch ef34839148d33b8d9cb03d56ffafdcdcbe9641a737211a50343d019132b169dd xsa342-4.13.patch $ DEPLOYMENT DURING EMBARGO ========================= Deployment of the patches and/or mitigations described above (or others which are substantially similar) is permitted during the embargo, even on public-facing systems with untrusted guest users and administrators. But: Distribution of updated software is prohibited (except to other members of the predisclosure list). Predisclosure list members who wish to deploy significantly different patches and/or mitigations, please contact the Xen Project Security Team. (Note: this during-embargo deployment notice is retained in post-embargo publicly released Xen Project advisories, even though it is then no longer applicable. This is to enable the community to have oversight of the Xen Project Security Team's decisionmaking.) For more information about permissible uses of embargoed information, consult the Xen Project community's agreed Security Policy: http://www.xenproject.org/security-policy.html -----BEGIN PGP SIGNATURE----- iQFABAEBCAAqFiEEI+MiLBRfRHX6gGCng/4UyVfoK9kFAl9p/ecMHHBncEB4ZW4u b3JnAAoJEIP+FMlX6CvZ+RAIAKhulm14Ze1LmVTCGKcTJ525DARSmzGdki4iX3ow qvQkV1B8TacFnuzZp1VfRnm5vRGBY/uXaFORw21Z/rWSRQ3xjgcazTsG0jhNQ8QG onH1JaxE26BfYu12oTSEKyTWWu1XSdrFTxWp07p79+qHvKGY6GtGRWGhkI6YNgkD X2TwRtt6GF6wRTq3PCc+7CGnn5jp7FRyJpI/2uiNZC6cL6lGUYNl9wgujSnefqQO 1sAZSc3DmvIuvFl4XWUeU7mH/6xL93sDN4vIrVllvcI9nEswqFwju6+SP76Pnkoh KBSYNk79QNlbBdXJwNmYxqp4sYpH/JYEm6+u2Zw1hxCMgM4= =EebG -----END PGP SIGNATURE----- Download attachment "xsa342.meta" of type "application/octet-stream" (2285 bytes) Download attachment "xsa342.patch" of type "application/octet-stream" (5749 bytes) Download attachment "xsa342-4.13.patch" of type "application/octet-stream" (5422 bytes)
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